Abstract

Free-space quantum communication with satellites opens a promising avenue for global secure quantum network and large-scale test of quantum foundations. Recently, numerous experimental efforts have been carried out towards this ambitious goal. However, one essential step - transmitting single photons from the satellite to the ground with high signal-to-noise ratio (SNR) at realistic environments - remains experimental challenging. Here, we report a direct experimental demonstration of the satellite-ground transmission of a quasi-single-photon source. In the experiment, single photons (∼0.85 photon per pulse) are generated by reflecting weak laser pulses back to earth with a cube-corner retro-reflector on the satellite CHAMP, collected by a 600-mm diameter telescope at the ground station, and finally detected by single-photon counting modules after 400-km free-space link transmission. With the help of high accuracy time synchronization, narrow receiver field-of-view and high-repetition-rate pulses (76 MHz), a SNR of better than 16:1 is obtained, which is sufficient for a secure quantum key distribution. Our experimental results represent an important step towards satellite-ground quantum communication.

Figures (5)

The scheme of the single-photon link from satellite to experimental setup installed at the Shanghai Observatory. CHAllenging Minisatellite Payload (CHAMP) was a German satellite launched July 15th, 2000 from Plesetsk, Russia and was used for atmospheric and ionospheric research, as well as other geoscientific applications. It was covered by 2 cm-diameter retroreflectors on each side. A train of pulses of 3 ps duration, 702 nm wavelength, 0.4 nJ of energy and 76 MHz repetition rate are coupled with the LRS pulses before being sent toward the transmitting telescope with aperture of 20 cm. The neutral density (ND) filter is used to control the weak pulse light, and the chopper is used to filter the backscattering noise. After the beam spreading, a fraction of the beam in the uplink path irradiates the satellite CHAMP. The corner cubes on the satellite retro-reflect back to the Earth a small portion of the photons in the laser pulse (downlink), which is the single-photon channel. Some of the photons in the downlink path are collected by the receiving telescope, a reflecting telescope with aperture of 60 cm, and detected by SPCMs, placed behind polarization measurement devices and spectral filters. The transmitting telescope and the receiving telescope are separated by a distance of 30 cm. All relevant events in the time domain are recorded by time measurement system TDC, and then referred to as coordinated universal time.

FOV of the detection system testing by scanning and recording the light intensity of the Polaris. The telescope was tracked to the Polaris, which could be seen as a fixed star. By changing the relative telescope tracking point, we record the different count rates of the changing position by SPCMs. Fig. 2(a) was the three-dimensional draw according to the counts. Fig. 2(b) was the fitting curve of FOV in the azimuth axial. Fig. 2(c) was the fitting curve of FOV in the zenith axial.The FOV of the receiving system, in both the azimuth and zenith axials, were measured to be less than 5″.

Plot of the range Rs between the satellite CHAMP and Shanghai Observatory. The round-trip time of the signal, directly proportional to Rs, exhibits strong and rapid variations during the satellite passage. The blue points are the LRS system’s records and the red line is the fitting curve on the survey data. The efficient time with maximal counts of single photons reflected from satellite and detected by SPCMs is shown in the gray box. The perigee height of CHAMP satellite that day was about 330km. The distance was ranging from 350 km to 400 km during our records of 15 s.

Histogram of all D values between expected and observed detections for CHAMP satellite.We summarized the D values numerically with 0.1ns as the unit. The peak of the histogram is centered at 0 ns, as expected. By Gaussian fitting, the full width at half maximum (FWHM) time accuracy was observed 1.35±0.03 ns.

Normalized intensity curve of horizontal and vertical polarization with the rotation of the telescope measured under the reference frame moving with the telescope. With a linearly polarized light incidenting without any compensation, the curve of horizontal (vertical) polarization show a sinesoid, as shown in curve H (V). With the polarization tracking system, the curve H′ and V′ show a nearly constant ratio. The maintenance of the polarization has a extinction ratio better than 100:1.